Polishing sheet and method of producing same

ABSTRACT

A polishing sheet suitable for precise polishing is produced by arranging polishing particles as a single particle layer on a base sheet and includes a base sheet and a polishing layer made of polishing particles and a binder resin for fixing the polishing particles that are arranged as a single particle layer. The polishing particles protrude partially from the surface of the binder resin, being covered or not covered with a thin film of the binder resin.

This application is a continuation of International Application No. PCT/JP2008/71070, filed Nov. 20, 2008 which claims priority on Japanese Patent Application 2007-303514 filed Nov. 22, 2007.

BACKGROUND OF THE INVENTION

This invention relates to a polishing sheet that may be cut into tapes and disks for a wide range of polishing from rough polishing to super-precision polishing, including polishing and abrading of materials such as metals, ceramics, glass and monocrystals, mirror polishing of the surfaces of magnetic hard disks and semiconductor substrates, precision final polishing of the surfaces of magnetic heads, lenses, etc., polishing of end surfaces of optical fibers, and surface finish polishing of automobile bodies, as well as a method of producing such polishing sheets.

In general, polishing sheets are produced by forming a polishing layer including abrading particles on a flexible base material sheet such as a plastic sheet, a woven or non-woven cloth or paper and by cutting it into an appropriate form such as a tape-like shape or a disk-shape for use.

The polishing layer is produced by applying slurry, which is a mixture of polishing (or abrading) particles and an adhesive binder, on the surface of a base material sheet, drying it and hardening it. It may be produced also by applying an adhesive binder on the base sheet, scattering polishing particles on the binder surface by a method such as the electrodeposition method after electrically charging them and drying the binder.

On such conventional polishing tapes, the polishing particles in the polishing layer have a multi-layered structure and these particles are intimately in contact mutually on the surface. Microscopically viewed, however, the surface structure is by no means uniform and aggregated lumps of polishing particles are protruding from the surface of the polishing layer at many locations. They tend to cause deep marks and scratches.

In high-precision polishing, fine polishing particles with regular size distribution must be used. If the particle diameters of a polishing material become small, the particles tend to aggregate easily and it becomes difficult to scatter the polishing particles uniformly inside the resin. If polishing particles are applied in their aggregated form, the surface roughness of the polishing tape becomes irregular, and marks are generated on the surface of the polished object. Polishing irregularities are also caused. Especially where the product characteristics are required to be improved such as the mirror polishing of the surface of a magnetic hard disk or a semiconductor substrate, the precision final polishing of the surface of a magnetic head, a lens, etc., and the polishing of end surfaces of optical fibers, improvements are being required in the smoothness and flatness of device surfaces at each production process.

It is becoming impossible to satisfy such requirements with conventional polishing tapes. In the case of polishing with fine polishing particles, the binder resin covers the surfaces of the polishing particles to increase the contact area between the polishing layer and the object being polished such that the friction with the polished object becomes large. Besides the marks caused by the aggregated clumps of the polishing particles as mentioned above, this causes deposition of the binder resin and makes it difficult to discharge debris generated by the polishing. Clogging is thereby caused and this adversely affects the useful lifetime of the polishing film.

In view of this problem, it has been known, as described, for example, in Japanese Patent Publication Tokkai 1-234169, to scatter the polishing particles in a binder resin for adjusting the solvent, the dispersant, their mixing ratio, the method of application and the amount of application such that a polishing layer is formed as a single-layer structure of the polishing particles attached to the sheet substantially without overlapping.

By this method, however, it is difficult to scatter the abrading material particles and apply them uniformly, and it is not possible to expose the cutting edges of the polishing particles on the binder resin surface.

As a method of exposing a portion of polishing particles from the polishing layer, on the other hand, it has been known, as described, for example, in Japanese Patent Publication Tokkai 2-243271, to apply the abrading particles together with the binder resin and to remove portions of the binder covering the particles by irradiating ultraviolet light after the binder has hardened such that the polishing particles come to be exposed on the polishing layer surface.

By this method, however, it takes a very long time to sufficiently remove the binder on the surface, adversely affecting the efficiency for mass production. There was also the problem of deterioration of the binder resin surface due to the long period of ultraviolet irradiation, causing the polishing particles to drop off.

According to another method, the binder resin is preliminarily applied to the surface of the base material, and an electric field is provided around this base material. After the polishing particles are charged, they are transported into the electric field for generating an electrostatic force on the polishing particles and to thereby cause them to become attached to the surface of the base material.

By such a method of charging polishing particles by means of an electric field formed between an electrode and the base material, however, there is a problem, in the case of very small particles of less than 10 μm, of becoming charged as aggregated clumps such that the single-particle distribution becomes difficult, as described, for example, in Japanese Patent Publication Tokkai 2003-340730.

It is therefore an object of this invention to provide a polishing sheet suitable for precision polishing, having polishing particles disposed as a single-particle layer as a polishing layer such that its flatness is improved and the generation of marks and scratches on the polished object will be reduced, as well as a method of producing such a polishing sheet.

SUMMARY OF THE INVENTION

In order to achieve the object described above, the present invention provides a method of producing a polishing sheet having polishing particles distributed in a single layer contained in a polishing layer formed on a surface of a base sheet, the method comprising an application step of applying a binder resin for fixing the polishing particles to the polishing layer, a charging step of charging a specified quantity of the polishing particles in a same polarity, a dispersing step of transporting and electrostatically dispersing the charged polishing particles to the base sheet having the binder resin applied thereto, and a hardening step of hardening the binder resin on the base sheet having the polishing particles dispersed thereto, wherein the binder resin is applied to the surface of the base sheet to a thickness no greater than the average diameter of the polishing particles.

It has been found to be possible by this production method to uniformly disperse polishing particles over the surface of a base sheet and even to control the necessary particle density. Since the thickness of the binder resin applied to the surface of the base sheet is made to be no greater than the average diameter of the polishing particles, furthermore, the polishing particles can be distributed in a single-layer formation, the polishing efficiency can be maintained high, and the generation of very small scratches and unevenness can be restrained.

The thickness of the binder resin applied to the surface of the base sheet is preferably two thirds or less and one tenth or more of the average diameter of the polishing particles.

It should be two thirds or less such that the polishing particles would not be buried inside the binder resin. It should be one tenth or more such that the quantity of the binder resin would be sufficient for preventing the polishing particles from dropping off from the binder resin.

It is further preferable that the polishing particles be preliminarily classified according to their uses such that a uniform polishing layer can be easily formed in a single-layer form.

The method is preferably carried out by sending the polishing particles into a metallic tube by means of a compressed gas and forming charged particles with a same polarity by friction by their contact with the inner wall of the tube.

The dispersing step is preferably carried out such that the polishing particles are charged to 1 kV-20 kV. It is because the repelling force would be too weak and it would become too difficult to form a single-layer formation if the charging potential were below 1 kV and the binder resin would scatter off and the surface would become damaged if it were above 20 kV.

The dispersing step may be carried out such that portions of the polishing particles become exposed from the surface of the binder resin such that portions of the polishing particles are exposed as cutting edges from the surface of the binder resin after this step. If the cutting edges are thus exposed on the surface, the speed of fabrication can be further increased.

The areal density of the polishing particles is preferably made to be 30%-95% on the polishing sheet. According to this invention, this can be adjusted easily by the number of times of emitting the particle even if the lots are small.

It is also preferable that dry powder with water content of 5% or less, and more preferably 1% or less, be used as the polishing particles. This has the effect of stabilizing the charge quantity and reducing the aggregation of small particles.

It is preferable to use an ultraviolet-setting resin as the binder resin. It is further preferable to use an epoxy resin as the binder resin for securely fixing the polishing particles.

The present invention also provides a polishing sheet comprising a base sheet and a polishing layer formed on a surface of this base sheet, wherein the polishing layer is formed with polishing particles and a binder resin that fixes the polishing particles, the polishing particles are distributed in a single layer, the polishing particles protrude in part from a surface of the binder resin or protrude from the surface of the binder resin with parts thereof covered with a thin film of the binder resin.

Since the polishing particles are distributed in a single-layer formation without being buried inside the binder resin, it is possible to accomplish polishing with a high efficiency while maintaining a long useful lifetime for the polishing sheet.

As explained above, a polishing sheet produced by the method of this invention can polish the end surface of optical fibers and the surface of a semiconductor substrate without forming any scratches while reducing steps even in the case of a structure with different materials and obtaining a flat and smooth surface.

Since polishing particles with similar diameters are used to form the polishing layer in a single-layer formation, individual particles work on the object to be polished effectively and hence the speed of polishing can be significantly improved.

Since the polishing particles are in a single-layer formation, furthermore, the amount of the particles that are used can be significantly reduced as compared to the prior art polishing sheets in a multi-layer formation and this contributes to the saving of the cost.

Since the polishing particles are dispersed onto the surface of the base sheet on which a specified amount of the binder resin is preliminarily applied while being charged by a specified amount, a production by a small lot becomes possible. The invention also has the merit of making it possible to freely adjust the areal density of the particles according to the purpose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B, together referred to as FIG. 1, are schematic sectional views of polishing sheets according to this invention.

FIG. 2 is a schematic sectional structural diagram of a charging-dispersing device for producing a polishing sheet of this invention.

FIGS. 3A, 3B and 3C, together referred to as FIG. 3, are each a microscopic photographic plan view of polishing particles dispersed according to this invention.

FIG. 4 is a SEM image of the surface of a polishing sheet produced by a method according to this invention.

FIG. 5 is a SEM image of the surface of a polishing sheet produced by a method of Comparison Example.

FIG. 6 is a schematic diagonal view of a test piece fabrication device using a polishing sheet according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described next with reference to the accompanying drawings.

FIGS. 1A and 1B are schematic sectional views of polishing sheets according to this invention, FIG. 2 is a schematic sectional structural diagram of a charge dispersing device for producing a polishing sheet of this invention, FIGS. 3A, 3B and 3C are each a microscopic photographic plan view of polishing particles dispersed according to this invention, FIG. 4 is a SEM image of the surface of a polishing sheet produced by a method according to this invention, FIG. 5 is a SEM image of the surface of a polishing sheet produced by a method of Comparison Example, and FIG. 6 is a schematic diagonal view of a test piece fabrication device using a polishing sheet according to this invention.

As shown in FIG. 1, a polishing sheet 30 according to this invention comprises a base sheet 31 made of a synthetic resin and a polishing layer 34 formed on the front surface of this base sheet 31. The polishing layer 34 has polishing particles 32 arranged in a single layer, these polishing particles 32 being fixed at their positions by a binder resin 33.

FIG. 1A shows a first structure of the polishing sheet according to this invention with polishing particles 32 arranged in a single particle layer having their surfaces thinly covered with a film of the binder resin 33. This is formed by the surface tension force of the binder resin, the polishing particles 32 being strongly fixed such that they will be prevented from dropping off. The portions of the binder resin film covering the edges of the polishing particles are extremely thin and peel of at the initial contact such that not only the abrading efficiency improves radically but also the binder resin can be prevented from becoming attached to the object being polished.

FIG. 1B shows a second structure of the polishing sheet according to this invention with the cutting edges of the polishing particles 32 exposed from the binder resin 33 such that the abrading efficiency is further improved.

Explained more in detail, portions of the cutting edges of the polishing particles 32 (or the portions having no resin film) can be made to appear from the surface of the binder resin 33 by adjusting the thickness of application of the binder resin 33 and the resin concentration. In doing so, it is desirable to make the heights of the cutting edges uniform. For this purpose, use is made of polishing particles that are classified and have uniform diameters. For carrying this out, it is preferable to apply the binder resin 33 to a thickness that is two thirds or less and one tenth or more of the average diameter of the polishing particles 32. If the thickness of the applied binder resin is greater than two thirds of the average diameter, more than 50% of the polishing particles will come to be covered by a film of the binder resin 33, and the abrading efficiency is adversely affected as the thickness of the film increases.

The areal density of the dispersed polishing particles is preferably 30% or more and 95% or less. If it is less than 30%, the abrading efficiency is adversely affected. If it exceeds 95%, the overlapping of particles takes place and it becomes difficult to distribute them in a single-layer formation.

Although classifications according to FIGS. 1A and 1B are separately shown, the formations shown in FIGS. 1A and 1B may be mixed. In other words, formations with the cutting edges of the polishing particles 32 exposed from the binder resin 33 and with the surfaces of the polishing particles 32 thinly covered by a film of the binder resin 33 may coexist.

Use as the base sheet 31 is made of a plastic film made of a synthetic resin because of the necessity for the resistance characteristics (high strength and heat resistance) against breakage and deformation due to mechanical forces applied during its use and deformation due to heat during its production as well as the necessity for flexibility. Depending on the purpose of use, however, paper, leather and rubber materials may be used. There is no particular limitation as to its shape. It may be in the shape of a sheet or a plate, and its surface may be flat, concave or convex.

Examples of material for such plastic film include polyester resins such as polyethylene terephthalate, polybuthylene terephthalate and polyethylene naphthalate, polyolefin resins such as polyethylene and polypropylene, polystyrene, vinyl chloride, polyvinyl alcohol, acryl resins with methacryl alcohol as principal component, and polycarbonates.

As a practical matter, however, it is preferable to use a plastic film made of polyethylene terephthalate as the base film because the handling of the film for producing a polishing film is easy.

There is no particular limitation on the thickness of the base sheet but it is preferably within the range of 5 μm or more and 100 μm or less, and more preferably within the range of 10 μm or more and 75 μm or less.

There is no particular limitation on the binder resin for fixing the polishing particles in the polishing layer. Ultraviolet-setting resins, electron beam-setting resins, visible light-setting resins, thermosetting resins, thermoplastic resins, and their mixtures may be used.

For the purpose of forming the polishing layer of a single-layer structure according to this invention, in particular, ultraviolet-setting resins and visible light-setting resins with low heat emission and low heating temperature are preferred for preventing displacements of the polishing particles in the hardening step of the production method.

If the binder resin is an ultraviolet-setting resin, epoxy-type, polyester-type or urethane-type epoxy acrylate, polyester acrylate and urethane acrylate as well as silicon acrylate and their mixtures containing a photoinitiator and a sensitizer are preferred.

There is no particular limitation on the polishing particles to be used for the present invention. For example, inorganic particles of alumina (Al₂O₃), silica (SiO₂), diamond (monocrystalline and polycrystalline), boron nitride (cBN) and silicon carbide (SiC) and organic particles of cross-linked acryl resins, cross-linked polystyrene resins, melamine resins, phenol resins, epoxy resins, urea resins and polycarbonate resins may be used.

Use of inorganic and organic particles may be appropriately selected according to the kind of the object to be polished, the degree of precision in the surface finishing or the protrusions to be removed and the shape of the protrusions.

There is no particulate limitation on the diameter of these polishing particles but particles with diameters in the range of 0.1 μm or more and 200 μm or less are preferred and those with diameters in the range of 0.5 μm or more and 50 μm or less are more preferred. If the diameter is less than 0.1 μm, it becomes difficult to arrange the polishing particles in a single layer formation. If the diameter exceeds 200 μm, on the other hand, the surface unevenness of the polishing particles becomes excessive, causing excessive scratches unsuitable for precision polishing.

It is preferable to use polishing particles that are classified according to the particle size distribution so as to make the height of the cutting edges of the polishing layer uniform because this will make it possible to form a flatter single-layer structure of polishing particles.

Next, a production method of a polishing tape is explained, inclusive of the principle of the dispersion of polishing particles and a charging-dispersing device.

The single-layer particle distribution of the polishing particles according to this invention is produced by a charging-dispersing device shown in FIG. 2.

In order to uniformly disperse a specified quantity of polishing particles over the surface of a base sheet preliminarily coated with a binder resin, these polishing particles are passed through a charging-dispersing device so as to become charged in a single polarity and dispersed. Since these charged particles repel one another and become mutually separated, they become attached to the binder resin without gathering together but by keeping appropriate distances among them as they are dispersed onto the surface of the base sheet. Since the charged particles do not become discharged immediately after they become attached, they do not aggregate again on the binder resin and keep their distances. Such charging-dispersing device is described below. The polishing particles may be charged positively or negatively.

An example of charging-dispersing device for dry polishing particles according to this invention will be explained with reference to FIG. 2 which is a sectional view schematically showing its structure including an electrostatic dispersion chamber 20 and a polishing particle supply unit 10.

The ceiling part of the dispersion chamber is provided with a dispersion nozzle 21 connected through a metallic pipe 16 for pressure-transporting the polishing particles to a particle adjustment chamber 11 inside the supply unit 10.

At a lower part inside the dispersion chamber is a stage 22 for dispersing the polishing particles, and a base sheet 23 is placed thereon. Below the stage 22 is an air discharge opening 24. There is also provided a collector opening (not shown) for collecting those of the polishing particles 32 which have failed to become attached to the base sheet 23 by dispersing.

The polishing particle supply unit 10 is provided with the adjustment chamber 11 and a quantity control box 13. The polishing particle adjustment chamber 11 contains therein a feeder unit (not shown) for controlling the collection of the polishing particles supplied from a supply hopper 12 and the quantity of the polishing particles transported into the pressure-transporting pipe 16. The polishing particles are led into the dispersion nozzle 21 of the dispersion chamber 20 through the pressure-transporting pipe 16.

The polishing particles 32 become charged inside the pressure-transporting pipe by contacting (by friction with) its inner wall, producing charged particles. These polishing particles repel one another and do not aggregate again since they are charged positively or negatively in a single polarity.

Methods of accelerating friction with the inner wall of the pressure-transporting pipe include, for preventing pressure-loss inside the pressure-transporting pipe, (a) providing the interior of the pressure-transporting pipe with the function of supplying a supplementary gas (dry air, nitrogen gas, etc.), (b) providing a negative pressure inside the pressure-transporting pipe, and (c) adding a plurality of branches to the pressure-transporting pipe for increasing the friction efficiency.

The charged polishing particles led to the dispersion nozzle 21 of the dispersion chamber 20 are blown onto the base sheet 23 together with a compressed gas discharged through gas nozzles provided around the dispersion nozzle 21.

Dry air and nitrogen gas from an ordinary high-pressure container may be used as the compressed gas.

The dispersion nozzle 21 is structured such that the charged particles will be uniformly dispersed onto the base sheet 23 as if blown by a spray gun. The dispersion nozzle 21 may comprise a plurality of openings having different angles and being adapted to open simultaneously or sequentially for dispersion. The charged polishing particles are blown towards the base sheet placed on the stage 22 without aggregating in the space inside the dispersion chamber 20, repelling one another and becoming deposited without aggregating again and by maintaining specified distances one from another.

For controlling the quantity of the dispersed polishing particles, a charge sensor 15 is provided near the pressure-transporting pipe 16 for detecting the quantity of the polishing particles being transported, and there is also provided a controller 14 containing a power source for controlling the device as a whole and a CPU at a lower part inside the polishing particle supply unit 10. The quantity of the dispersed polishing particles can thus be controlled by monitoring the charges on the dispersed particles by means of the charge sensor 15 and feeding back this monitored quantity of the charges.

Thus, the density of the polishing particles on the surface of the base sheet 23 can be determined by obtaining the quantity for each dispersing emission and by counting the number of times of emission. The charges on the particles vary, depending on the kind of the material of the polishing particles, the particle diameter and shape, and are preliminarily determined and adjusted.

The polishing particles, thus electrostatically dispersed, come to be attached to the binder resin in a single-particle layer formation without overlapping and by maintaining a set distance among them since they are charged with a same polarity.

The dispersion density of the polishing particles on the base sheet can be adjusted by varying the quantity of each emission and the number of times of emission.

There is no particular limitation on the shape of the base sheet. Dispersion may be made on a plate-like material, a sheet or a tape. In the case of a tape-shaped object, the production can be continuously carried out while running the tape.

Test Example

A polishing sheet embodying this invention was prepared as follows.

(1) A base sheet of polyethylene terephthalate (PET) of 75 μm in thickness was used. Al₂O₃ and diamond with average diameter of 5 μm were used as polishing particles.

(2) As the binder resin, Adeka HCX200-25 (resin: PGM 17% by weight) was used as coating solution.

(3) Polycrystalline diamond particles with average diameter of 5 μm were used as the polishing particles.

The coating solution thus prepared was applied to the surface of the base sheet by a bar coater method to a thickness of 50% of the average particle diameter (or 2.5 μm).

The polishing particles were charged and dispersed by using a charging-dispersing device shown in FIG. 2. The quantity of the particles per emission was 0.4 g/min and the areal density of the polishing particles was adjusted by the number of times of emission.

FIGS. 3A, 3B and 3C are examples of the condition of diamond particles with average diameter of 5 μm dispersed at a low density (FIG. 3A), at a medium density (FIG. 3B) and at a high density (FIG. 3C) by the number of times of emission. FIG. 3 shows that the particles are uniformly dispersed.

The areal density of the polishing particles on the polishing sheet to be used for the evaluation (to be described below) was about 50% (for FIG. 3B).

For the dispersion, the diameter of the dispersion nozzle was 5 mm and a carrier gas was discharged at a rate of 3 kg/cm² for 5 seconds. This discharge was repeated for 10 times. The charging potential at this time was 5 kV.

For hardening the binder resin, used was made of an ultraviolet irradiator J-Cure 1500 of Jatec Corporation.

For the irradiation, the output was 1 kW, wavelength was 365 nm (at the peak), the cumulative quantity of light was 600 mJ/min, and the irradiation time (transportation by a belt) was 0.5 m/min.

A polishing sheet was produced by carrying out a post-cure process thereafter at 130° C. for 30 minutes (in an isothermal tank).

FIG. 4 shows an image of a polishing sheet produced by a method according to this embodiment (Test Example) of the invention as observed by a scanning electron microscope. As shown in FIG. 4, the cutting edges of the polishing particles are distributed in a single-particle layer formation and, although covered by a thin binder resin layer, are exposed from the surface of the binder resin.

Comparison Example

A polishing sheet of Comparison Example was produced by using polishing particles which are identical to those used for the production of the polishing sheet of Test Example and by a conventional method (of applying a mixture of polishing particles with a binder resin onto a base sheet).

FIG. 5 is an image of the surface of a polishing sheet produced under these conditions of Comparison Example as observed by a scanning electron microscope.

Evaluation

The polishing sheets of aforementioned Test Example and Comparison Example were individually used to carry out ball bearing fabrication tests and comparisons were made regarding the polished quantity for a specified length of polishing time, the central line average surface roughness (Ra) and the maximum surface roughness (Rmax).

The tests using the polishing sheets were carried out on steel balls (SUJ-2) with a diameter of 4 mm of ball bearings as test objects by using a polishing device for fabrication shown in FIG. 6. The fabrication device 40 is adapted to have a polishing sheet 43 of this invention pasted on a rotatable lapping plate 42, to have a steel ball 44 as described above as an object of polishing fixed to a jig 45 and to have a specified load 47 applied from above onto a polishing head 41 provided to a principal shaft 46.

The fabrication test was carried out by rotating the lapping plate 42 with a polishing sheet 43 pasted thereto, causing the polishing head 41 with the steel ball 44 fixed thereto to contact the surface of the polishing sheet 43 pasted onto the lapping plate 42 with a specified load and to move a specified distance from a central position to an outer peripheral position on the lapping plate 42 at a specified speed. The fabrication process is started and ended automatically by moving up and down an arm 48 supported around a supporting point 49. After the fabrication process is completed, the steel ball 44 is removed from the jig 45, the steel ball 44 is weighed and the change (or the reduction) in its weight serves as the polished quantity (or the stock removal). Five different steel balls were used for the test and the average value from them was used for the evaluation.

The conditions of the test were as follows:

(1) Load: 500 g

(2) Diameter of the lapping plate: 8 inches

(3) Rotary speed of the lapping plate: 300 rpm

(4) Distance of motion from the central position to the peripheral position: 100 mm

(5) Polishing time: 12 seconds

The average surface roughness Ra and the maximum surface roughness Rmax of the polished steel balls were measured by a surface roughness meter (SURFCON 480A produced by Tokyo Seimitsu Kabushiki Kaisha).

Comparison Results

The comparison results are shown below in Table 1.

TABLE 1 Average surface Maximum surface roughness (Ra) roughness (Rmax) Stock removal (μm) (μm) (mg/min) Test Example 0.024 0.149 8.20 Comparison 0.037 0.256 7.20 Example

As shown in Table 1, the polishing by the polishing sheet of Test Example is distinctively effective as compared to that by the polishing sheet of Comparison Example both in the average surface roughness (Ra) and the maximum surface roughness (Rmax) by polishing for a specified length of time. With the polishing sheet of Test Example, furthermore, a higher stock removal can be achieved for finishing the surface at a lower surface roughness.

Since the amount of polishing particles to be used for Test Example is less than 1/50 of that in the case of Comparison Example, the present invention serves to reduce the material cost significantly. 

1. A method of producing a polishing sheet having polishing particles distributed in a single-layer formation contained in a polishing layer formed on a surface of a base sheet; said method comprising: an application step of applying a binder resin for fixing said polishing particles to said polishing layer; a charging step of charging a specified quantity of said polishing particles in a same polarity; a dispersing step of transporting and electrostatically dispersing said charged polishing particles to said base sheet having said binder resin applied thereto; and a hardening step of hardening said binder resin on said base sheet having said polishing particles dispersed thereto; wherein said binder resin is applied to the surface of said base sheet to a thickness no greater than the average diameter of said polishing particles.
 2. The method of claim 1 wherein the thickness of said binder resin applied to the surface of said base sheet is two thirds or less and one tenth or more of the average diameter of said polishing particles.
 3. The method of claim 1 wherein said polishing particles are preliminarily classified according to their uses.
 4. The method of claim 1 wherein said charging step comprises sending said polishing particles into a metallic tube and forming charged particles with said same polarity by frictional charging.
 5. The method of claim 1 wherein said polishing particles are charged to 1 kV-20 kV in said dispersing step.
 6. The method of claim 1 wherein portions of said polishing particles are exposed as cutting edges from the surface of said binder resin after said dispersing step.
 7. The method of claim 1 wherein said polishing particles have an areal density of 30%-95% on said polishing sheet.
 8. The method of claim 1 wherein dry powder with water content of 5% or less is used as said polishing particles.
 9. The method of claim 1 wherein an ultraviolet-setting resin is used as said binder resin.
 10. A polishing sheet comprising a base sheet and a polishing layer formed on a surface of said base sheet, wherein said polishing layer is formed with polishing particles and a binder resin that fixes said polishing particles, said polishing particles are distributed in a single layer, said polishing particles protrude in part from a surface of said binder resin or protrude from the surface of said binder resin with parts thereof covered with a thin film of said binder resin. 